MRC is a file format for electron density that has become industry standard in Cryo-electron microscopy, where the result of the technique is a three-dimensional grid of voxels each with a value corresponding to density of electrons. It was developed by the MRC Laboratory of Molecular Biology.
The MRC format is supported by almost every molecular graphics suite that supports volumetric data. The major packages include:
Visual molecular dynamics
UCSF Chimera
IMOD